Superabsorbent polymers

ABSTRACT

Polymer compositions which are highly absorbent to aqueous electrolyte solutions are prepared by copolymerization of an ampholytic monomer having the formula of ##STR1## where one and only one of the substituted groups R 1 , R 2 , R 3  and R 4  must be a vinyl group, the rest can be the same or different and can be hydrogen or a C 1  -C 3  alkyl group; R 5  is a phenyl group; n is ≧1 and ≦6; and q is 0 or 1; and an olefin monomer having an ethylenic structure as well as by graft copolymerization of at least an olefin monomer having an ethylenic structure and an ampholytic monomer having the formula disclosed above onto a main polymer selected from the group consisting of polysacchardies and polyolefins. Also disclosed is a process for absorbing large quantity of aqueous electrolyte solution employing the polymer compositions.

This application is a divisional of application Ser. No. 07/873,259,filed Apr. 24, 1992, now U.S. Pat. No. 5,225,506.

FIELD OF THE INVENTION

The present invention relates to polymers capable of absorbing aqueouselectrolyte solutions.

BACKGROUND OF THE INVENTION

Polymers for absorbing aqueous electrolyte solutions are used innumerous commercial and industrial applications. For example, polymersare used to improve the water absorbency of paper towels and disposablediapers.

Though known water absorbing polymers are highly absorbent to deionizedwater, they are dramatically less absorbent to aqueous electrolytesolutions such as salt water, brine, and urine. For example, hydrolyzedcrosslinked polyacrylamide absorbs 1,024 grams of deionized water pergram of polymer, but only 25 grams of synthetic urine per gram ofpolymer. Crosslinked polyacrylate absorbs 423 grams of deionized waterper gram of polymer, but only 10 grams of synthetic urine per gram ofpolymer. Hydrolyzed crosslinked polyacrylonitrile absorbs 352 grams ofdeionized water per gram of polymer, but only 25 grams of syntheticurine per gram of polymer. Analogous starch grafted copolymers generallyhave very poor absorbency to synthetic urine.

It would be a valuable contribution to the art to develop polymers withhigh absorbency to aqueous electrolyte solutions. It also would be avaluable contribution to the art to develop biodegradable graftcopolymers which were highly absorbent to aqueous electrolyte solutions.The market for these types of copolymers is large and the uses arenumerous. Therefore, seemingly small improvements in the absorbencytranslate into large savings in the quantity of copolymer required toabsorb these liquids and large savings to the consumer.

SUMMARY OF THE INVENTION

Thus, it is an object of the present invention to provide polymers whichare highly absorbent to aqueous electrolyte solutions. It is also anobject of the present invention to provide a process for preparing thepolymers having high absorbency to aqueous electrolyte solutions.Another object of the present invention is to provide biodegradablepolymers which are highly absorbent to aqueous electrolyte solutions. Afurther object of the present invention is to provide a method of usingthe polymers of the present invention for absorbing an aqueouselectrolyte solution.

Further objects, features, and advantages of the present invention willbe readily apparent to those skilled in the art upon reading thedescription of the invention which follows.

According to a first embodiment of the present invention, a polymerhaving a high absorbency to aqueous electrolyte solutions is providedwhich comprises repeating units derived from: (a) an ampholytic monomerhaving the formula of: ##STR2## where one and only one of thesubstituted groups R₁, R₂, R₃ and R₄ must be a vinyl group, the rest canbe the same or different and is a hydrogen or a C₁ -C₃ alkyl group; R₅is a phenyl group; n is ≧1 and ≦6; and q is 0 or 1; (b) at least onecomonomer selected from the group consisting of acrylamide,methacrylamide, acrylonitrile, acrylic acid, methacrylic acid, alkalisalts of acrylic acid, alkali salts of methacrylic acid,2-methacryloyloxyethyltrimethylamine, 2-acrylamido-2-methylpropanesulfonic acid, alkali salts of 2-acrylamido-2-methylpropane sulfonicacid, 2-methacryloyloxyethane sulfonic acid, alkali salts of2-methacryloyloxyethane sulfonic acid,3-methacrylamidopropyltrimethylamine,3-methacrylamidopropyldimethylamine, 2-methacryloyloxyethyldiethylamine,styrene sulfonic acid, alkali salts of styrene sulfonic acid, andN-vinyl-2-pyrrolidone; and (c) at least one crosslinking agent which hasat least two polymerizable olefinic functionalities wherein the olefinicfunctionalities are suitable for crosslinking.

According to a second embodiment of the present invention, a graftcopolymer having a high absorbency to aqueous electrolyte solutions isprepared by contacting a main polymer chain selected from the groupconsisting of a polysaccharides and a polyolefins under graftpolymerization conditions with

(a) at least one olefin monomer selected from the group consisting ofacrylamide, methacrylamide, acrylonitrile, acrylic acid, methacrylicacid, alkali salts of acrylic acid, alkali salts of methacrylic acid,2-methacryloyloxyethyldiethylamine, 2-acrylamido-2-methylpropanesulfonic acid, alkali salts of 2-acrylamido-2-methylpropane sulfonicacid, 2-methacryloyloxyethane sulfonic acid, alkali salts of2-methacryloyloxyethane sulfonic acid,3-methacrylamidopropyltrimethylamine,3-methacrylamidopropyldimethylamine, 2-methacryloyloxyethyldiethylamine,styrene sulfonic acid, alkali salts of styrene sulfonic acid, andN-vinyl-2-pyrrolidone; and

(b) an ampholytic monomer having the formula of: ##STR3## where one andonly one of the substituted groups R₁, R₂, R₃ and R₄ must be a vinylgroup, the rest can be the same or different and can be hydrogen or a C₁-C₃ alkyl group; R₅ is a phenyl group; n is ≧1 and ≦6; and q is 0 or 1.

According to a third embodiment of the present invention, a process forabsorbing an aqueous electrolyte solution comprises contactingsuperabsorbent polymer selected from the group consisting of a graftcopolymer which is prepared by contacting a main polymer chain selectedfrom the group consisting of polysaccharides and polyolefins under graftpolymerization conditions with (a) at least one olefin monomer selectedfrom the group consisting of acrylamide, methacrylamide, acrylonitrile,acrylic acid, methacrylic acid, alkali salts of acrylic acid, alkalisalts of methacrylic acid, 2-methacryloyloxyethyldimethylamine,2-acrylamido-2-methylpropane sulfonic acid, alkali salts of2-acrylamido-2-methylpropane sulfonic acid, 2-methacryloyloxyethanesulfonic acid, alkali salts of 2-methacryloyloxyethane sulfonic acid,2-methacryloyloxyethyldiethylamine, 3-methacrylamidopropyldimethylamine,styrene sulfonic acid, alkali salts of styrene sulfonic acid, andN-vinyl-2-pyrrolidone; and (b) copolymerizing therewith an ampholyticmonomer having the formula of: ##STR4## where one and only one of thesubstituted groups R₁, R₂, R₃ and R₄ must be a vinyl group, the rest canbe the same or different and can be hydrogen or a C₁ -C₃ alkyl group; R₅is a phenyl group; n is ≧1 and ≦6; and q is 0 or 1; and an absorbentpolymer which is prepared by copolymerization of (a) an ampholyticmonomer having the formula of: ##STR5## where one and only of thesubstituted groups R₁, R₂, R₃ and R₄ must be a vinyl group, the rest canbe the same or different and can be hydrogen or a C₁ -C₃ alkyl group; R₅is a phenyl group; n is ≧1 and ≦6; and q is 0 or 1; (b) at least onecomonomer selected from the group consisting of acrylamide,methacrylamide, acrylonitrile, acrylic acid, ethacrylic acid, alkalisalts of acrylic acid, alkali salts of methacrylic acid,2-methacryloyloxyethyltrimethylamine, 2-acrylamido-2-methylpropanesulfonic acid, alkali salts of 2-acrylamido-2-methylpropane sulfonicacid, 2-methacryloyloxyethane sulfonic acid, alkali salts of2-methacryloyloxyethane sulfonic acid,2-methacryloyloxyethyldiethylamine, 3-methacrylamidopropyldimethylamine,styrene sulfonic acid, alkali salts of styrene sulfonic acid, andN-vinyl-2-pyrrolidone; and (c) at least one crosslinking agent which hasat least two polymerizable olefinic functionalities wherein the olefinicfunctionalities are suitable for crosslinking; with an aqueouselectrolyte solution.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a polymer that is highly absorbent toaqueous electrolyte solutions. Typical aqueous electrolyte solutionsinclude, but are not limited to the group consisting of tap water, saltwater, brine and urine. The term "polymer" used here generically refersto a polymer having two or more different monomers, i.e. copolymers,terpolymers, tetrapolymers, etc., and includes those prepared bycopolymerization of an effective amount of each of the followingmonomers to produce a polymer that has the above-described properties:(a) an ampholytic monomer having the formula of: ##STR6## where at leastand only one of the substituted groups R₁, R₂, R₃ and R₄ must be a vinylgroup, the rest can be the same or different and can be hydrogen or a C₁-C₃ alkyl group; R₅ is a phenyl group; n is ≧1 and ≧6; and q is 0 or 1;(b) at least one comonomer selected from the group consisting ofacrylamide, methacrylamide, acrylonitrile, acrylic acid, methacrylicacid, alkali salts of acrylic acid, alkali salts of methacrylic acid,2-methacryloyloxyethyltrimethylamine, 2-acrylamido-2-methylpropanesulfonic acid, alkali salts of 2-acrylamido-2-methylpropane sulfonicacid, 2-methacryloyloxyethane sulfonic acid, alkali salts of2-methacryloyloxyethane sulfonic acid,2-methacryloyloxyethyldiethylamine, 3-methacrylamidopropyldimethylamine,styrene sulfonic acid, alkali salts of styrene sulfonic acid, andN-vinyl-2-pyrrolidone; and (c) at least one crosslinking agent which hasat least two polymerizable olefinic functionalities wherein the olefinicfunctionalities are suitable for crosslinking.

Unless otherwise indicated, the term "alkali salts" is generically usedin this application to mean salts containing ammonium cation and alkalimetal cations such as lithium, sodium and potassium.

The presently preferred ampholytic monomer of the present invention is ahydroxide form in aqueous solution and is selected from the groupconsisting of 1-vinyl-3-(3-sulfopropyl)-imidazolium hydroxide,1-vinyl-3-(4-sulfobutyl)imidazolium hydroxide,1-vinyl-2-methyl-3-(3-sulfopropyl)imidazolium hydroxide,1-vinyl-2-methyl-3-(4-sulfobutyl)imidazolium hyroxide,1-vinyl-3-(2-sulfobenzyl)imidazolium hydroxide,2-vinyl-3-(3-sulfopropyl)imidazolium hydroxide,2-vinyl-3-(4-sulfobutyl)imidazolium hydroxide,4(5)-vinyl-1-(3-sulfopropyl sulfobutyl)imidazolium hydroxide,4(5)-vinyl-1-(4-sulfobutyl)imidazolium hydroxide,1-methyl-2-vinyl-3-(3-sulfopropyl)imidazolium hydroxide and1-vinyl-3-(4-sulfobutyl)imidazolium hydroxide. The presently preferredampholytic monomer is 1-vinyl-3-(3-sulfopropyl)-imidazolium hydroxide(hereinafter referred to VSPIH).

The olefinic comonomers can include, but are not limited to the groupconsisting of acrylamide, methacrylamide, acrylonitrile, acrylic acid,methacrylic acid, alkali salts of acrylic acid, alkali salts ofmethacrylic acid, 2-methacryloyloxyethyltrimethylamine,2-acrylamido-2-methylpropane sulfonic acid, alkali salts of2-acrylamido-2-methylpropane sulfonic acid, 2-methacryloyloxyethanesulfonic acid, alkali salts of 2-methacryloyloxyethane sulfonic acid,2-methacryloyloxyethyldiethylamine, 3-methacrylamidopropyldimethylamine,styrene sulfonic acid, alkali salts of styrene sulfonic acid,N-vinyl-2-pyrrolidone and combinations of two or more thereof. Thesecomonomers are believed to be commercially available.

Suitable crosslinking agents can include but are not limited to thegroup consisting of N,N-diallylmethacrylamide, diallylamine,N,N-bisacrylamidoacetic acid, N,N'-bisacrylamidoacetic acid methylester,N,N'-methylenebisacrylamide (methylene-bis-acrylamide),N,N-benzylidenebisacrylamide, allylacrylate, diisopropenylbenzene,diallyl succinate, ethylene glycol diacrylate, diallylacrylamide,divinylbenzene, and combinations of two or more thereof. All thesesuitable crosslinking agents are commercially available. The presentlypreferred crosslinking agent is N,N'-methylenebisacrylamide.

The polymers of the present invention are generally prepared by mixingthe various monomers in desired stoichiometric ratios in aqueoussolution and then initiating the free-radical copolymerization. Thecopolymerization of a ampholytic monomer with an olefinic comonomer anda crosslinking agent can be achieved by any of the well knownfree-radical polymerization techniques in solution, suspension, oremulsion environment. Well known azo compounds commonly employed toinitiate free radical polymerization reactions include2,2'-azobis(N,N'-dimethylisobutyramidine) dihydrochloride,azobisisobutyronitrile, 4,4'-azobis(4-cyanovaleric acid),2,2'-azobis(2,4-dimethyl(4-methoxyvaleronitrile),2,2'-azobis(2,4-dimethylvaleronitrile),2,2'-azobis(2-amidinopropane)-dihydrochloride,2-t-butylazo-2-cyano-4-methoxy-4-methylpentane, and2-t-butylazo-2-cyano-4-methylpentane, and 4-t-butylazo-4-cyanovalericacid. Well known inorganic peroxide compounds commonly employed toinitiate free radical polymerization reactions include hydrogenperoxide, alkali metal persulfates, alkali metal perborates, alkalimetal perphosphates, and alkali metal percarbonates. Well known organicperoxide compounds commonly employed to initiate free radicalpolymerization reactions include lauryl peroxide,2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane,t-butylperoxyprivilate, t-butylperoctoate, p-methane hydroperoxide, andbenzoylproxide. The compound t-butylhyponitrite is a well known alkylhyponitrite commonly employed to initiate free radical polymerizationreactions. Furthermore, ultraviolet light is commonly employed toinitiate free radical polymerization reactions. In addition, such othermethods of copolymerization as would have occurred to one skilled in theart may be employed, and the present invention is not limited to theparticular method of preparing the polymer set out herein.

These inventive copolymers of the present invention containing anolefinic comonomer with amine, amide, nitrile, carboxylic acid, orsulfonic acid functionalities or crosslinking agent with amide, nitrile,carboxylic acid, or sulfonic acid functionalities can optionally be atleast partially hyrolyzed and/or neutralized by heating with aqueousbase such as aqueous sodium hydroxide or aqueous potassium hydroxide.The degree of hydrolysis and/or neutralization can be controlled bystoichiometrically limiting the amount of base relative to the amount ofamide, nitrile, carboxylic acid, and sulfonic acid functionalities. Ifthe hydrolysis is carried out under acidic conditions, the amide andnitrile functionalities can be converted to carboxylic acidfunctionalities without neutralizing the carboxylic acid or sulfonicacid functionalities of the polymer.

The polymers of the present invention can also optionally be saponified.The term "saponified" used herein, unless otherwise indicated, is thesame as at least partially hydrolyzed and/or neutralized of the nitrileor amide functionalities by heating the polymer with an aqueous base.The presently preferred base is selected from the group consisting oflighium hydroxide, sodium hydroxide, potassium hydroxide, ammoniumhydroxide, calcium hydroxide, and mixtures thereof. The presentlypreferred base is sodium hydroxide. Hydrolysis of these functionalitiescan be carried out under acidic or basic conditions. Under basicconditions, it generally also includes neutralization of carboxylic acidand sulfonic acid functionalities. The degree of hydrolysis and/orneutralization can be controlled by stoichiometrically limiting theamount of base relative to the amount of amide, nitrile, carboxylicacid, and sulfonic acid functionalities.

The mole percent of the ampholytic monomer of the present inventionranges from about 1 to about 60, preferably from about 2 to about 50,and most preferably 3 to 30. The mole percent of the comonomers(s)ranges from about 40 to about 99, preferably from about 50 to about 98,and most preferably from 70 to 97. The total mole percent of theampholytic monomer and the comonomer equals to 100 mole percent. Thecrosslinking agent is provided in an amount effective to produce ahighly absorbent copolymer. The mole percent of the crosslinking agentcan be from about 0.001 to about 5, preferably from about 0.01 to about2.5, and most preferably from 0.02 to 1 based on the total mole percentof the polymer.

The second embodiment of the present invention provides grafted polymersthat are highly absorbent to aqueous electrolyte solutions. Graftedpolymers as used herein are polymers of one or more species of monomersconnected to a main chain as a side chain, exclusive of branch point onthe main chain. Side chains of a grafted polymer are distinguished fromthe main polymer chain by the monomer constitution of the side chaini.e., the side chains comprise units derived from at least one speciesof monomer different from those that supply the units of the mainpolymer chain. The main polymer chain as utilized in the presentinvention are homopolymeric and copolymeric polymer such aspolysaccharides, polypropylene, polyethylene and other polyolefins. Theside chains are formed of at least one olefinic comonomer and anampholytic monomer.

The term "graft copolymerization" is used herein, unless otherwiseindicated, to mean a copolymer which results from the formation of anactive site or sites at one or more points on the main chain of apolymer molecule other than its end and exposure to at least one othermonomer. The graft polymers of the present invention are prepared bygraft copolymerization of an effective amount of each of the followingcomponents onto a first polymer (main polymer chain) to produce a highlyabsorbent polymer: at least one comonomer selected from the groupconsisting of acrylamide, methacrylamide, acrylonitrile, acrylic acid,methacrylic acid, alkali salts of acrylic acid, alkali salts ofmethacrylic acid, 2-methacryloyloxyethyldiethylamine,2-acrylamido-2-methylpropane sulfonic acid, alkali salts of2-acrylamido-2-methylpropane sulfonic acid, 2-methacryloyloxyethanesulfonic acid, alkali salts of 2-methacryloyloxyethane sulfonic acid,2-methacryloyloxyethyldiethylamine, 3-methacrylamidopropyldimethylamine,styrene sulfonic acid, alkali salts of styrene sulfonic acid, andN-vinyl-2-pyrrolidone to form a first grafted polymer; and thereaftergraft copolymerizing therewith an ampholytic monomer having the formulaof: ##STR7## where one and only one of the substituted groups R₁, R₂, R₃and R₄ must be a vinyl group, the rest can be the same or different andcan be hydrogen or a C₁ -C₃ alkyl group; R₅ is a phenyl group; n is ≧1and ≦6; and q is 0 or 1.

It should be noted, however, the ampholytic monomer can also be graftcopolymerized onto the first polymer to form a first grafted copolymerand thereafter graft copolymerized therewith at least one comonomerselected from the group described above.

Polymers which may be used as main chains in the practice of the presentinvention include polysaccharides and polyolefins. Polysaccharidessuitable for the practice of the present invention include starches,celluloses and glycogens. Common sources of cellulose include but arenot limited to cotton, linen, rayon, wood pulp and cellulose xanthine.Currently, cotton gauze is preferred. Suitable starches included swollenamylose and amylopectin starches. For the practice of the presentinvention, these starches should be swollen by heating the starch inwater to substantially dissolve the starch granules. Preferably starchesused in the present invention will have less than 30 weight percentamylose based on the weight of the dry starch before graftcopolymerization. The preferred starch for use in grafting is solublestarch flour within the range of from about 0 to about 20 weight percentamylose content. Suitable polyolefins include polypropylene andpolyethylene. The polypropylene suitable for use as main polymer chainincludes polypropylene homopolymers, polypropylene copolymer andpolypropylene block-copolymers. The polyethylene suitable for use as amain polymer chain includes polyethylene homopolymer, polyethylenecopolymers and polyethylene block-copolymers. Preferably the syntheticpolymers listed above will be utilized in the form of filaments or thinsheets so that a high surface area to mass will be provided for graftingtherewith the comonomers and ampholytic monomer. Filaments utilized forgrafting will preferably have a denier ranging from about 1 to about 20denier and most preferably from in the range of about 1 to 8 denier.

The term "monomer" is used generically, unless otherwise indicated, tomean monomers, comonomers, termonomers, tetramonomers, etc. The term"comonomer" is used generically, unless otherwise indicated, to meanmonomers, comonomers, termonomers, tetramonomers, etc. for polymerswherein there are at least two different monomer.

The scope of the olefinic comonomer and the ampholytic monomer are thesame as those described above.

The polymers of the second embodiment of the present invention aregenerally prepared in a two step process, though a single graftcopolymerizing step or more than two grafting and polymerizing steps maybe advantageously employed. The purpose of the two step process is toprovide a first grafted polymer wherein the grafted comonomer sidechains are more reactive to the polymerization of the ampholyticmonomer. Some systems may be reactive enough so that a two step processis not necessary to provide grafted copolymers which are highlyabsorbent to aqueous electrolyte solutions. Alternatively, the multiplestep process may be advantageously employed to control the proportionsof monomers and relative lengths of the block copolymer chains by graftcopolymerizing the various monomers in the desired stochiometric ratiosat the appropriate step of the process.

In the preparation of polysaccharide grafted polymers, it is preferredthat as a first step, at least one of the comonomers is graftcopolymerized onto a polysaccharide, to produce a first polysaccharidegraft copolymer. Then in a second step, the ampholytic monomer is graftcopolymerized onto the polysaccharide or the ampholytic monomer ispolymerized onto the grafted comonomer side chains. At the second or anysubsequent graft copolymerizing step, the ampholytic monomer may becopolymerized with at least one other comonomer. At the second step orany subsequent graft copolymerizing step, the ampholytic monomer may becopolymerized with at least one comonomer which has a polymerizableolefinic functionality selected from the group consisting of acrylamide,methacrylamide, acrylonitrile, acrylic acid, methacrylic acid, alkalisalts of acrylic acid, alkali salts of methacrylic acid,2-methacryloyloxyethyldiethylamine, 2-acrylamido-2-methylpropanesulfonic acid, alkali salts of 2-acrylamido-2-methylpropane sulfonicacid, 2-methacryloyloxyethane sulfonic acid, alkali salts of2-methacryloyloxyethane sulfonic acid and N-vinyl-2-pyrrolidone.

The graft copolymerization of the ampolytic monomer may require a highertemperature than the graft polymerization of some of the othercomonomers. Therefore, for the graft copolymerization at temperatures inthe range of from about 0° C. to about 90° C. and preferably in therange of from about 40° C. to about 70° C. Those skilled in the art willrecognize that the temperatures at which the polymerization is carriedout should be varied to allow the various monomers and comonomer toreact completely within a reasonable period of time for the method ofpolymerization utilized.

Most graft copolymerization methods for olefinic monomers involve thecreation of reactive sites (for example free-radicals) on the mainpolymer chain. These reactive sites then serve to initiate thecopolymerization of the other monomers onto the main copolymer chain.Free-radicals reactive sites on the main chain generally are produced byhigh energy radiation or chemical initiation. A common chemical meansfor creating these free-radicals within polysaccharide polymers andpolypropylene polymers is with a chemical oxidation-reduction system.Examples of such oxidation-reduction systems include but are not limitedto oxidation-reduction systems selected from the group consisting ofceric ammonium nitrate/nitric acid, ceric ammonium sulfate/sulfuricacid, potassium permanganate/oxalic acid, hydrogen peroxide/ferrousalkali salts, hydrogen peroxide/ascorbic acid and amine/persulfate.Common irradiation means for producing free radicals on the main polymerchain is by utilizing a gamma radiation source (i.e. cobalt to) or anelectron beam.

The copolymerization of the ampholytic monomer with the olefiniccomonomer onto the grafted comonomer side chains can be achieved by anyof the well known free-radical polymerization techniques in solution,suspension, or emulsion environment. The techniques are the same asthose described above.

Optionally, the inventive graft polymers of the present invention can becrosslinked with a suitable crosslinking agent. The crosslinking agentsand the crosslinking techniques are the same as those described above.

The relative amount of the main polymer chain to the total weight of thecomonomer and ampholytic monomer can be chosen to provide a graftedpolymer of variable absorbency. However, the main polymer chaingenerally constitutes in the range of from about 1 to about 50 weightpercent, and preferably from 5 to 30 weight percent of the total weightof comonomers, ampholytic monomer and main polymer chain present. Themole percent of comonomer and ampholytic monomer which may be graftcopolymerized onto the main polymer chain is in the range of from about75 to about 98 and from about 2 to about 25, respectively; andpreferably from 80 to 97 and from 3 to 20, respectively, based on thetotal moles of the comonomers and ampholytic ion pair totaling a 100percent.

According to the third embodiment of the present invention, a processfor absorbing aqueous electrolyte solutions comprises contacting anabsorbent polymer selected from the group consisting of a polymerprepared by copolymerizing: (a) an ampholytic monomer having the formulaof: ##STR8## where one and only one of the substituted groups R₁, R₂, R₃and R₄ must be a vinyl group, the rest can be the same or different andcan be hydrogen or a C₁ -C₃ alkyl group; R₅ is a phenyl group; n is ≧1and ≦6; and q is 0 or 1; (b) at least one comonomer selected from thegroup consisting of acrylamide, methacrylamide, acrylonitrile, acrylicacid, ethacrylic acid, alkali salts of acrylic acid, alkali salts ofmethacrylic acid, 2-methacryloyloxyethyltrimethylamine,2-acrylamido-2-methylpropane sulfonic acid, alkali salts of2-acrylamido-2-methylpropane sulfonic acid, 2-methacryloyloxyethanesulfonic acid, alkali salts of 2-methacryloyloxyethane sulfonic acid,2-methacryloyloxyethyldiethylamine, 3-methacrylamidopropyldimethylamine,styrene sulfonic acid, alkali salts of styrene sulfonic acid, andN-vinyl-2-pyrrolidone; and (c) at least one crosslinking agent which hasat least two polymerizable olefinic functionalities wherein the olefinfunctionalities are suitable for crosslinking; and a graft polymerprepared by contacting a first polymer selected from the groupconsisting of polysaccharides, polyolefins with: (a) at least one olefinmonomer selected from the group consisting of acrylamide,methacrylamide, acrylonitrile, acrylic acid, methacrylic acid, alkalisalts of acrylic acid, alkali salts of methacrylic acid,2-methacryloyloxyethyldiethylamine, 2-acrylamido-2-methylpropanesulfonic acid, alkali salts of 2-acrylamido-2-methylpropane sulfonicacid, 2-methacryloyloxyethane sulfonic acid, alkali salts of2-methacryloyloxyethane sulfonic acid,2-methacryloyloxyettyldiethylamine, 3-methacrylamidopropyldimethylamine,styrene sulfonic acid, alkali salts of styrene sulfonic acid, andN-vinyl-2-pyrrolidone; and (b) copolymerizing therewith an ampholyticmonomer having the formula of: ##STR9## where one and only one of thesubstituted groups R₁, R₂, R₃ and R₄ must be a vinyl group, the rest canbe the same or different and can be hydrogen or a C₁ -C₃ alkyl group; R₅is a phenyl group; n is ≧1 and ≦6; and q is 0 or 1.

Typical aqueous electrolyte solutions include but are not limited toelectrolyte solutions selected from the group consisting of tap water,salt water, brine, and urine. For the purpose of this invention, tapwater is defined to have an electrolyte concentration of less than 500ppm of dissolved electrolytes, urine is defined to have an electrolyteconcentration of from greater than 500 ppm to at most 10,000 ppm ofdissolved electrolytes, salt water is defined to have an electrolyteconcentration from greater than 10,000 ppm to at most 34,000 ppm andbrine is defined to have an electrolyte concentration of from greaterthan 34,000 ppm to the saturation point of the solution.

The scope of the absorbent polymer and grafted polymer is the same asthat described above.

The following examples are provided to illustrate the advantages of thepresent invention and are not intended to unduly limit the presentinvention.

COMPARATIVE EXAMPLE I

This comparative example shows the absorbency of known crosslinkedpolymers.

The crosslinked polymers were prepared by mixing the monomers in theproportions given in Table I in an aqueous solution of deionized water.The monomers were present in 30-40 weight % relative to the amount ofdeionized water. The free radical polymerization was initiated withcommercially available 2,2'-azobis(N,N'-dimethylisobutyramidine)dihydrochloride. The reaction mixture was then degassed by bubblingnitrogen gas through the mixture for 15 minutes. The amount of the azofree-radical initiator employed was 0.1 mole percent, based on the totalmoles of the monomers. The reaction temperature was maintained between20°-35° C. for 24 hours. The reactions produced transparent or cloudyhard gels of the crosslinked polymers. A large volume (1,000 ml for 6gram gel) of deionized water was added to the polymer product and thepolymers were allowed to swell for about 24 hours. The swelled polymerswere dried in a forced air convection oven at 74° F. The dried polymerswere then mechanically blended into a powder.

Some of the crosslinked polymers were hydrolyzed or neutralized with astrong base such as aqueous sodium hydroxide or aqueous potassiumhydroxide. The degree of hydrolysis or neutralization was controlled bystoichiometrically limiting the amount of base relative to the amount ofamide, nitrile, or carboxylic acid functionalities. For these examples,a stoichiometric excess of the amount of base was used. A suspension of1 gram of the polymer in about 20 ml of 0.5N aqueous sodium hydroxidewas heated to 95° C. until a light golden-yellow color was obtained. Themixture was then transferred to a dialysis bag with a molecular weightcut-off of 12,000-14,000 and dialyzed exhaustively against distilledwater until the viscous polymer gel had reached pH 7. This viscouspolymer gel was then poured into a plastic dish and dried in a forcedair convection oven at 74° C. The dried polymers were then mechanicallyblended to a powder.

The crosslinked polymers were then tested for deionized water absorptionand synthetic urine absorption. About 1 liter of deionized water orsynthetic urine was added to 0.1 to 0.5 gram of the dried polymer andallowed to stand for 24 hours. The polymer was then separated from theexcess unabsorbed liquid by screening through a 100 mesh per inchstainless steel sieve. The absorbency was determined by weighing theisolated polymer containing the absorbed liquid and subtracting theweight of the dry polymer.

The absorbency was measured in units of grams of liquid per grams ofpolymer. The synthetic urine was prepared by dissolving 0.64 gram CaCl₂,1.14 gram MgSO₄.7H₂ O, 8.20 gram NaCl, and 20.0 gram urea into 1000 gramdeionized water. Several of the polymers were tested two or three times,and the experimental error was within plus or minus 2-5 percent. Thissmall experimental error was largely caused by gel blocking and minordiffusion problems that prevented the aqueous liquid from contactingwith all the polymer. The results are shown in Table I.

                                      TABLE I                                     __________________________________________________________________________    Control Data for Known Crosslinked Polymers                                             Mole Percent   Mole Ratio*                                                                              g/g**                                     RUN #                                                                              AMPS ®                                                                         AM AN AA X-AA  LINK   XOH DIW SU                                    __________________________________________________________________________     1   --   100                                                                              -- -- --    0.05   NO   17 15                                     2   --   100                                                                              -- -- --    0.05   YES 1024                                                                              25                                     3   --   100                                                                              -- -- --    0.05   YES 364 40                                     4   --   100                                                                              -- -- --    0.20   NO   13   12.5                                 5   --   100                                                                              -- -- --    0.20   YES 295 16                                     6   --   -- 100                                                                              -- --    0.05   YES 608 46                                     7   --   -- 100                                                                              -- --    0.10   NO   0  0                                      8   --   -- 100                                                                              -- --    0.10   YES 414 42                                     9   --   -- 100                                                                              -- --    0.20   YES 352 25                                    10   --   -- -- 100                                                                              --    0.20   NO   21 11                                    11   --   -- -- 100                                                                              --    0.20   Neu.sup.+                                                                         423 10                                    12   --   -- -- -- 100                                                                              (K)                                                                              0.05   NO  669 57                                    13   --   -- -- -- 100                                                                              (Na)                                                                             0.05   NO  505 41                                    14   --   13 -- -- 87    0.05   NO  --  65                                    15   3    13 -- -- 84    0.05   NO  350 38                                    16   3    20 -- -- 77    0.05   NO  417 47                                    17   6    13 -- -- 81    0.05   NO  738 56                                    18   6    26 -- -- 68    0.05   NO  533 47                                    19   6    -- -- -- 94    0.05   NO  488 55                                    20   10   13 -- -- 77    0.05   NO  570 59                                    21   20   13 -- -- 67    0.05   NO  624 62                                    22   100  -- -- -- --    0.05   NO  Soluble                                   __________________________________________________________________________     AMPS ® = 2acrylamido-2-methylpropane sulfonate (Note: AMPS ® is a     trademark of Lubrizol for 2acrylamido-2-methylpropane sulfonic acid).         AM = Acrylamide                                                               AN = Acrylonitrile                                                            AA = Acrylic Acid                                                             XAA = Sodium Acrylate or Potassium Acrylate                                   LINK = Methylenebis-acrylamide Crosslinking Agent                             XOH = Basic Hydrolysis and/or Neutralization with aqueous NaOH or KOH         DIW = Deionized Water                                                         SU = Synthetic Urine                                                          *mole ratio = mole of the crosslinking agent per 100 mole of the              ampholytic monomer and the comonomer                                          **g/g = absorbency units of gram aqueous liquid per gram dried polymer        +Neu = Neutralized                                                       

The data in Table I demonstrates that although known crosslinkedpolymers are highly absorbent to deionized water, they are dramaticallyless absorbent to aqueous electrolyte solutions such as salt water andurine. Polysaccharide grafted polymers, however, according to theirinherent nature, are normally much less absorbent to aqueous liquids.The polysaccharide substrate, which comprises a large portion of thematerial, is very poorly absorbent to aqueous liquids of all kinds. Thiscontrol data can be used to show that the polysaccharide grafted VSPIHcopolymers of the present invention can effectively compete with theseknown crosslinked polymers and exceed the absorbency of these knowncrosslinked polymers. Furthermore, these known crosslinked polymers havequestionable biodegradeability.

Known polymer compositions include crosslinked polyacrylamide, partiallysaponified crosslinked polyacrylamide, crosslinked polyacrylonitrile,partially saponified crosslinked acrylonitrile, crosslinked polyacrylicacid, neutralized crosslinked polyacrylic acid, crosslinkedpolyacrylate, and polymers thereof with sodium2-acrylamido-2-methylpropane sulfonate. The best of these known polymersabsorb up to 65 g of synthetic urine per g of polymer, and most of theknown polymers absorb much less than 50 g of synthetic urine per of g ofpolymer.

COMPARATIVE EXAMPLE II

The data in Table II demonstrates that although commercially availablewater absorbing materials are highly absorbent to water, they are alsodramatically less absorbent to aqueous electrolyte solutions such assalt water and urine. The commercially available water absorbingmaterials tested include poly(co-acrylamide-co-acrylic acid) graftedonto starch, a commercial acrylamide polymer sold under the trademark"Water Grabber"® ("Water Grabber" is a trademark of F. P. Products,Inc.), "LUVS"® diaper absorbent ("LUVS" is a trademark of Procter &Gamble Co.), "Pampers"® diaper absorbent ("Pampers" is a trademark ofProcter & Gamble Co.), and Favor 960"® (Stockhausen, Inc.). The best ofthese known materials absorb up to about 56 grams of urine per gram ofabsorbing material, and most of the known polymers absorb much less than40 grams of urine per gram of absorbing material.

The commercially available materials were tested for absorbency toaqueous liquids according to the method employed in Example I.

                  TABLE II                                                        ______________________________________                                        Control Data for Commercial Materials                                                                DIW    SU                                              EXP #  commercial Material    g/g*                                            ______________________________________                                        1      Commercial Starch-g-Poly(AM-AA)                                                                     345      37                                      2      Water Grabber ® (AM Copolymer)                                                                  440      34                                      3      Luvs ® Diaper Absorbent                                                                         191      16                                      4      Pampers ® Diaper Absorbent                                                                      171      12                                      5      Favor 960 ®       369      56                                      ______________________________________                                         g = graft                                                                     AM = Acrylamide                                                               AA = Acrylic Acid                                                             DIW = Deionized Water                                                         SU = Synthetic Urine                                                          *g/g = absorbency units of gram aqueous liquid per gram dried polymer    

EXAMPLE III

The homopolymers of the ampholytic monomer comprising VSPIH with 2.4 and0.05 mole percent methylene-bis-acrylamide crosslinking agent was testedfor their absorbency to deionized water and synthetic urine according tothe method employed in Example I. The absorbency of homopolymers is verypoor. See Table III. The absorbency to deionized water is about 0.77gram to 3.00 grams water per grams of homopolymer, and only 1.07 and4.00 grams of synthetic urine per gram of homopolymer, respectively.

                  TABLE III                                                       ______________________________________                                        Control Data for Homopolymer of Ampholytic Monomer                                                   Ag. Solvent Absorbency                                 VSPIH       MBA*       (g/g**)                                                Run   Mole %    Mole Ratio DIW      SU                                        ______________________________________                                        1     100       2.5        0.77     1.07                                      2     100       0.05       3.00     4.00                                      ______________________________________                                         VSPIH = 1vinyl-3-(3-sulfopropyl)imidazolium hydroxide                         MBA = Methylenebis-acryalamide                                                *Mole Ratio = mole of the crosslinking agent per 100 moles of the             ampholytic monomer and comonomer                                              DIW = Deionized water                                                         SU = Synthetic Urine                                                          **g/g = Absorbency with 4 gram aqueous liquid per gram dried polymer     

EXAMPLE IV

The polymers of this present invention were prepared according to themethod described in Example I, except that the inventive polymers wereprepared by mixing the monomers in the proportions given in Table IV.Some of these copolymers which contain amide and nitrile functionalitieswere hydrolyzed with an aqueous base such as sodium hydroxide orpotassium hydroxide. This treatment resulted in slightly yellowcoloration in the hydrolyzed products. All the copolymers were testedfor absorbency in DIW and SU. The results given in Table IV demonstratethat these ampholytic copolymers exhibit significantly improvedabsorbency in the aqueous electrolyte solutions, such as SU, over theabsorbency of the control polymer listed in Table I and the commercialpolymers listed in Table II. Taking an absorbency of 35 g of SU per g ofpolymer for example (Run 1), it shows an improvement of about 118% overthe SU absorbency of the equivalent control polymer Run 5 (Table I) orthe "Luvs Diaper Absorbent" (Table II) which has apparently similar gelstrength. Comparing an absorbency of 56 g of SU per g of the bestcommercial polymer, FAVOR 60 (Table II), the experimental polymer ofapparently similar gel strength (Run 9) exceeds this absorbency in SU byabout 71%.

                                      TABLE IV.sup.a                              __________________________________________________________________________    Experimental Data for VSPIH Copolymers                                                                        Absorbency                                    Mole Percent         Mole Ratio*                                                                              (g/g)                                         Run #                                                                             VSPIH                                                                             AM AN AA X-AA                                                                              X-LINK XOH DIW SU                                        __________________________________________________________________________    1   10  90 -- -- --  0.20   Yes 587 35                                        2   10  90 -- -- --  0.10   Yes 397 38                                        3   10  90 -- -- --  0.05   Yes 820 46                                        4    6  94 -- -- --  0.10   Yes 378 44                                        5    3  97 -- -- --  0.10   Yes 378 44                                        6    3  97 -- -- --  0.03   Yes 1400                                                                              55                                        7   10  90 -- -- --  0.03   Yes 1900                                                                              95                                        8   10  90 -- -- --  0.03   Yes 1900                                                                              95                                        9   15  85 -- -- --  0.03   Yes 1323                                                                              96                                        10   3  -- 97 -- --  0.06   Yes --  25                                        11   6  -- 94 00 00  0.06   Yes --  43                                        12  10  -- 90 -- --  0.06   Yes --  45                                        13  15  -- 85 -- --  0.06   Yes --  65                                        14   3  20 -- -- 77  0.05   No  276 65                                        15   6  20 -- -- 74  0.05   No  312 79                                        16  10  20 -- -- 70  0.05   No  200 70                                        __________________________________________________________________________     .sup.a See Table I footnotes for abbreviations.                          

EXAMPLE V

The polysaccharide grafted VSPIH copolymers of this present invention inTable V were generally prepared according to the following two stepprocedure.

About 5 grams of reagent grade soluble starch was added to 50 ml ofdeionized water. While stirring under inert nitrogen atmosphere, thesoluble starch slurry was heated to 95° C. for 1 hr. after which theheat was removed and the stirred soluble starch slurry was allowed tocool to room temperature, about 22° C. A solution of 0.17 gram cericammonium nitrite in 1.5 ml of 1N nitric acid was added to the cooledstirring soluble starch slurry. After about 5 minutes, the olefiniccomonomer (0.0659-0.0848 moles) was then added to the soluble starchslurry mixture. The particular comonomer and relative mole percent addedfor each of the tested polysaccharide grafted VSPIH copolymers isprovided in Table V. The mixture was stirred under inert nitrogenatmosphere for two hours.

The mixture was then heated to 60° C., at which point a solution of 0.17gram ceric ammonium nitrate in 1-5 ml of 1N nitric acid was added to themixture. After about 1 minute, a 70 wt % solution of the ampholyticmonomer dissolved in deionized water was added to the warmed mixture.The particulate VSPIH monomer and relative mole percent added for eachof the tested polysaccharide grafted VSPIH copolymers is provided inTable V. This new mixture was stirred under nitrogen at 60° C. foranother 6-8 hours.

The pH of the mixture was adjusted to between pH 4 and pH 5. The solidcrude polysaccharide grafted VSPIH copolymers was obtained byevaporating the aqueous solvent in a forced air convection ovenmaintained at 74° C. The crude grafted polymer was washed by boiling indimethylformamide to remove any non-grafted acrylonitrile homopolymer.It was then thoroughly washed with water to remove any water solublepolymer. The purified grafted material was finally washed with ethanoland dried in a vacuum oven at 70° C. for 24 hours. The dried polymerswere then mechanically blended to a powder form. The yield ofpolysaccharide grafted VSPIH copolymers was typically in excess of 55 wt% based on the total weight of the soluble starch, comonomer, andampholytic monomer.

Some of the inventive polysaccharide grafted VSPIH copolymers containingan olefinic comonomer with amine, amide, nitrile, carboxylic acid orsulfonic acid functionalities were hydrolyzed and/or neutralized with anaqueous base such as aqueous sodium or potassium hydroxide according tothe method described in Example I. The polymers were recovered by dryingin a forced air convection oven at 74° C. The dried polymers were thenmechanically blended to a powder form.

The polysaccharide grafted VSPIH copolymers were tested for theirabsorbency according to the method employed in Example I.

                  TABLE V.sup.a                                                   ______________________________________                                        Comparative Data with Graft Copolymer                                                                               Aqueous Solvent                                                 Yield         Absorbency                              Run  VSPIH     AN       Weight        g/g                                     #    Mole %    Mole %   Percent XOH   DIW  SU                                 ______________________________________                                        1    --        100      50      Yes   783  36                                 2    12        88       55      Yes   900  77                                 3    19.7      80.3     55      Yes   712    73.5                             ______________________________________                                         .sup.a See footnotes of Table I for abbreviations.                       

As can be seen in table V, the grafted copolymers have much highersynthetic urine absorbency (77 and 73.5 g/g, runs 2 and 3, respectively)than control (Run 1 of Table V and Runs 1-2 of Table III).

Thus, the present invention is well adapted to carry out the objects andobtain the ends and advantages mentioned above as well as those inherenttherein. While presently preferred embodiments have been described forpurposes of this disclosure, numerous changes will be apparent to thoseskilled in the art. Such changes are encompassed within the spirit ofthe invention as defined by the appended claims.

That which is claimed is:
 1. A graft copolymer prepared by contacting a main polymer selected from the group consisting of polysaccharides, polyolefins, and mixtures thereof; under graft polymerization conditions with:(a) at least one olefinic monomer having functionality selected from the group consisting of amine, amide, nitrile, carboxylic acid, sulfonic acid, and mixtures thereof; and (b) an ampholytic monomer having the formula of: ##STR10## where one and only one of the substituted groups R₁, R₂, R₃ and R₄ must be a vinyl group, the rest can be the same or different and can be hydrogen or a C₁ -C₃ alkyl group; R₅ is a phenyl group; n is ≧1 and ≦6; and q is 0 or
 1. 2. A graft copolymer according to claim 1 wherein said olefinic monomer is first graft polymerized onto said main polymer to produce an olefin grafted main polymer chain and thereafter said ampholytic monomer is graft polymerized onto said olefin grafted main polymer chain.
 3. A graft copolymer according to claim 1 wherein said ampholytic monomer is first graft copolymerized onto said main polymer to produce an ampholytic monomer grafted main polymer chain and thereafter said olefin monomer is graft copolymerized onto said ampholytic monomer grafted main polymer chain.
 4. A graft copolymer according to claim 1 wherein a mixture of said olefinic monomer and said ampholytic monomer is graft polymerized onto said main polymer chain.
 5. A graft copolymer according to claim 1 wherein said olefinic monomer is selected from the group consisting of acrylamide, methacrylamide, acrylonitrile, acrylic acid, methacrylic acid, alkali salts of acrylic acid, alkali salts of methacrylic acid, 2-methacryloyloxyethyldiethylamine, 2-acrylamido-2-methylpropane sulfonic acid, alkali salts of 2-acrylamido-2-methylpropane sulfonic acid, 2-methacryloyloxyethane sulfonic acid, alkali salts of 2-methacryloyloxyethane sulfonic acid, 3-methacrylamidopropyltrimethylamine, 3-methacrylamidopropyldimethylamine, 2-methacryloyloxyethyldiethylamine, styrene sulfonic acid, alkali salts of styrene sulfonic acid, N-vinyl-2-pyrrolidone, and mixtures thereof.
 6. A graft copolymer according to claim 1 wherein contemporaneously with said graft polymerization a crosslinking agent is copolymerized therewith, wherein said crosslinking agent is provided in an amount effective to produce a highly absorbent graft copolymer.
 7. A graft copolymer according to claim 6 wherein said crosslinking agent is N,N'-methylenebisacrylamide.
 8. A graft copolymer according to claim 1 wherein said main polymer is a polysaccharide selected from the group consisting of starch, cellulose, glycogen, and mixtures thereof.
 9. A graft copolymer according to claim 8 wherein said polysaccharide is starch.
 10. A graft copolymer according to claim 1 wherein said ampholytic monomer is selected from the group consisting of 1-vinyl-3-(3-sulfopropyl)-imidazolium hydroxide, 1-vinyl-3-(4-sulfobutyl)imidazolium hydroxide, 1-vinyl-2-methyl-3-(3-sulfopropyl)imidazolium hydroxide, 1-vinyl-2-methyl-3-(4-sulfobutyl)imidazolium hydroxide, 1-vinyl-3-(2-sulfobenzyl)imidazolium hydroxide, 2-vinyl-3-(3-sulfopropyl)imidazolium hydroxide, 2-vinyl-3-(4-sulfobutyl)imidazolium hydroxide, 4(5)-vinyl-1-(3-sulfopropyl sulfobutyl)imidazolium hydroxide, 4(5)-vinyl-1-(4-sulfobutyl)imidazolium hydroxide, 1-methyl-2-vinyl-3-(3-sulfopropyl)imidazolium hydroxide, 1-vinyl-3-(4-sulfobutyl)imidazolium hydroxide, and mixtures thereof.
 11. A graft copolymer according to claim 10 wherein said ampholytic monomer is 1-vinyl-3-(3-sulfopropyl)imidazolium hydroxide.
 12. A graft copolymer according to claim 6 wherein said olefinic monomer is acrylonitrite.
 13. A graft copolymer according to claim 1 wherein said grafted copolymer comprises: (a) from about 1 to about 50 weight % of said polysaccharide; and (b) from about 75 to about 98 mole % of said olefinic monomer and from about 2 to about 25 mole % of said ampholytic monomer based on total mole % of said olefinic monomer and ampholytic monomer equaling 100%.
 14. A graft copolymer according to claim 13 wherein said grafted copolymer comprises: (a) from 5 to 30 weight % of said polysaccharide; and (b) from 80 to 97 mole % of said olefinic monomer and from 3 to 20 mole % of said ampholytic monomer based on total mole % of said olefinic monomer and said ampholytic monomer equaling 100%.
 15. A graft copolymer according to claim 13 wherein said grafted copolymer comprises: (a) from 5 to 30 weight % of said starch; and (b) from 80 to 97 mole % of acrylonitrile and from 3 to 20 mole % of said 1-vinyl-3-(3-sulfopropyl)imidazolium hydroxide based on total mole % of acrylonitrile and 1-vinyl-3-(3-sulfopropyl)imidazolium hydroxide equaling 100%.
 16. A graft copolymer according to claim 1 wherein said graft copolymer is partially hydrolyzed. 